New Relic is a monthly service that is activated by installing their plugin. Just head to their site, create a free account, and they will send a welcome email with the plugin install link, a yml file, and instructions.
The free version will track CPU, DB, and Rails Response times/load and display them in a nifty graph for those visual types.
Five Runs is a gem that you add to you environment.rb. The response times are displayed in a div that is added to the top of the page in dev mode.
After testing them on two different sites, we found new relic to work nicer to see where problems are.
In dev mode, you can open up a new tab, them navigate to your
localhost/newrelic, and the 100 most recent bottlenecks are displayed.
In production, a very tiny, tiny amount of processing power will send the data to your account at newrelic.com.
From there you can login and see all the data in various forms.
Depending on your budget and project needs, there are monthly service fee brackets with different levels of functionality.
Its worth checking out!